Long day plants transformed with phytochrome characterized by altered flowering response to day length

ABSTRACT

A long day plant cultivated for commercial production of flowering-shoots, flowering pots, flowers, seeds or fruits is provided. The long day plant overexpresses a phytochrome protein such that the flowering-shoots, flowering pots, flowers, seeds or fruits thereof develop under substantially shorter natural days and lower sun irradiance than those required for development of the flowering-shoots, flowering pots, flowers, seeds or fruits in a similar long day plant not over expressing the phytochrome protein.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to long day plants characterized byaltered flowering response to day length and to methods of generatingsame. More particularly, embodiments of the present invention relate tolong day plants cultivated for commercial production offlowering-shoots, flowering pots, flowers, seeds or fruits which arecharacterized by altered responsiveness to day length, and to methods ofproducing such plants.

Many physiological processes in plants are known to be synchronized withthe daily light cycle; some of these processes respond to thephotoperiod length within this cycle, a phenomenon known asPhotoperiodism.

Photoperiodism is characteristic of plants belonging to differenttaxonomic groups, such as monocotyledonous, dicotyledonous, perennials,annuals, bulb or corm forming plants.

Flowering response is one of many processes affected by photoperiodism.Depending on the plant species, flowering response can be affected in aqualitative (i.e. induction of flowering) or quantitative (i.e.acceleration of the flowering process) manner.

Plants in which the flowering process is induced or accelerated by daylength can be divided according to a critical, maximal or minimal daylength needed to induce or accelerate such flowering. For example,flowering of short day plants is induced or accelerated under aphotoperiod shorter than a critical length whereas flowering of long dayplants is induced or accelerated under a photoperiod longer than acritical length (Bemier et al, 1981).

There are plants in which distinct stages of the flowering processdiffer in the type of response to day length. In such plants asequential application of varying day length (short followed by long orlong followed by short) is needed to induce or accelerate theirflowering process (Halevy A. H., 1985). Furthermore, there are long dayplants which, in addition to a need for a critical day length, alsorespond to light intensity (light photon fluence rate) during one ormore stages of their flowering process. In such plants, a higher lightintensity will shorten the critical day length for the floweringresponse (Vince-Prue, 1994).

Plant photoreceptors which participate in the photoperiodism response tolight, include the well characterized group of phytochromes.Phytochromes are a group of related proteins which are encoded by atleast five divergent genes and which function as photoreceptors of lightin a wavelength maximum of 660 ηm (Red=R) and 730 ηm (Far-Red=FR) of thephotospectrum. The phytochromes respond to the ratio of R to FR in thelight spectrum and to the photon fluence rate in this spectrum.

Phytochrome A (which is encoded by PHY A gene) and phytochrome B (whichis encoded by PHY B gene) are members of this group. Phytochromes A andB are activated by Red light, while phytochrome A is also degraded underRed light and activated by Far-Red light. This difference in activationby light is associated with different modes of action for phytochromes Aand B (Casal et al, 1996). In general, phytochromes may function asintegral light-switchable components of transcriptional regulatorcomplexes, permitting continuous and immediate sensing of changes inlight signals directly at target gene promoters so as to allow controlof the pattern of gene expression involved in determiningphotomorphogenic processes, including flowering (Murtas and Millar,2000).

Phytochrome A or B expression levels affect various plantphotomorphogenic responses, including flowering induction (Whitelam andDevlin, 1997). Phytochromes A and B participate in various processes,which enable the plant to sense a natural day cycle of light and dark.For example, phytochromes are responsible for measuring the length of alight period within a daily cycle and for responding to the transitionbetween light and dark periods (Thomas and Vince-Prue, 1995: Somer etal, 1998: Samach and Coupland, 2000: Murats and Millar, 2000).

The role of phytochrome in regulating plant growth is of considerablecommercial value in agriculture. As such, several prior art documentsdescribe phytochrome-overexpressing plants, which present agronomictraits of commercial value.

For example, U.S. Pat. No. 5,268,526 to Hershey et al. describes thepreparation of a transgenic plant overexpressing phytochrome of amonocotyledonous plant origin. The transgenic plant described by Hersheyet al, exhibits a variety of useful agronomic traits such as reducedapical dominance, semidwarfism, increased shade tolerance, or dark greencolor.

U.S. Pat. No. 5,945,579 to Smith describes the generation of transgenictobacco plants overexpressing phytochrome A, in which over expressionconfers an ability to undergo proximity-conditional dwarfing.

The role of phytochromes A and B in regulating the flowering response today-length was studied in mutants lacking phytochrome expression (nullmutants of either PHY A or PHY B) and in plants overexpressingphytochrome A or B. The role of phytochromes A and B in the regulationof the flowering response to day-length in long day plants was studiedunder artificial growth conditions which differed from naturalconditions in light spectrum, light intensity and temperature. Undersuch artificial growth conditions a PHY A null mutant flowered laterthan wild type plants grown under long day conditions; although such PHYA deficiency did not change the flowering response to short-dayconditions (Johanson et al, 1994: Weller et al, 1997: Reed et al, 1994:Whitelam and Harberd, 1994).

Flowering of Arabidopsis (a long day plant) overexpressing phytochrome Awas studied also under tissue culture conditions which includedartificial sterile agar medium, high humidity, constant temperature of23° C., and fluorescent lighting of 200 μmol m⁻² s⁻¹ followed by dayextension with 10 μmol m⁻² s⁻¹ from incandescent lamps. Under suchconditions, overexpression of phytochrome A caused early flowering underlong and short-day illumination conditions (Bagnall et al, 1995).

Under artificial growth conditions phytochrome B deficient Arabidopsismutants (PHY B null mutants) flowered earlier than wild type plants,independent of day-length conditions (Putterill et al, 1995: Whitelamand Harberd, 1994: Goto et al, 1991: Reed et al, 1993), whileoverexpression of phytochrome B in Arabidopsis enhanced flowering onlyin tissue culture grown plants, both under long and short-day artificiallighting conditions (Bagnall et al, 1995).

Although various prior art studies regarding the effect of phytochromeoverexpression on flowering describe long day plants in which earlyflowering was induced under artificial short or long day lightingconditions, such early flowering traits were induced under artificiallighting and growth conditions.

Artificial light conditions typically include white light provided fromfluorescent lamps and/or far-red rich light provided from incandescentlamps. White light lacks the far-red spectrum while light from bothincandescent and fluorescent lamps lacks the ultra violet spectrum.Furthermore, the intensity ratio between the different wavelengthscomprising the artificial light used by these studies differs from thatof sunlight.

In contrast, natural light (sunlight) is composed of various wavelengthsof nearly equal intensities. In addition, sunlight varies in wavelengthcomposition and intensity during dawn and dusk, cloudy or dusty days andaccording to the sun position at different seasons of the year.

In addition, under natural lighting conditions a temperature-lightintensity relationship exists since high temperature is typicallyaccompanied by high sun irradiance intensity and vice versa, whereasunder artificial conditions, temperature and light intensity are notinterdependent.

In most of the prior art studies described above, the artificial lightintensities were much lower than sun light intensity, in the range of60-200 μmol m⁻² s⁻¹ and were kept constant and irrespective oftemperature throughout the experimentally induced short day conditions.

In addition, the growth conditions employed in the tissue cultureexperiments were extremely different from commercial growth conditionsalso in the root zone medium and the atmosphere composition within thetube.

Thus, although prior art long-day transgenic plants overexpressingphytochrome A are of some potential commercial value, at presentlong-day plants can only be made to flower earlier in the year underartificial lighting conditions and not under natural conditionsdesirable for commercial application of such early flowering trait.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, phytochrome-overexpressing long-day plantscultivated for commercial production of flowering-shoots, floweringpots, flowers, seeds or fruits characterized by altered responsivenessto day length.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided along day plant cultivated for commercial production of flowering-shoots,flowering pots, flowers, seeds or fruits; the long day plantoverexpressing a phytochrome protein in at least a portion of it'scells, such that the flowering-shoots, flowering pots, flowers, seeds orfruits thereof develop under substantially shorter days than thatrequired for development of the flowering-shoots, flowering pots,flowers, seeds or fruits in a similar long day plant not overexpressingthe phytochrome protein.

According to another aspect of the present invention there is provided amethod of modulating a responsiveness of a long day plant to day length,the method comprising the step of overexpressing a phytochrome proteinin at least a portion of the cells of the long day plant underconditions such that flowering-shoots, flowering pots, flowers, seeds orfruits of the long day plant develop under substantially shorter daysthan those required for development of the flowering-shoots, floweringpots, flowers, seeds or fruits in a similar long day plant notoverexpressing the phytochrome protein.

According to further features in preferred embodiments of the inventiondescribed below, the long day plant overexpressing the phytochromeprotein is derived from a commercial plant, plant derived tissue or aplant cell transformed with an exogenous expression cassette foroverexpressing the phytochrome protein.

According to still further features in the described preferredembodiments the commercial plant, plant derived tissue or a plant cellis stably or transiently transformed with the expression cassette foroverexpressing the phytochrome protein.

According to still further features in the described preferredembodiments the expression cassette forms a part of a nucleic acidconstruct selected from the group consisting of a DNA construct or anRNA construct.

According to still further features in the described preferredembodiments the exogenous expression cassette includes a phytochrome Aor a phytochrome B encoding sequences.

According to still further features in the described preferredembodiments the exogenous expression cassette also includes a promotersequence for directing expression of the phytochrome A or thephytochrome B encoding sequences in plant tissue.

According to still further features in the described preferredembodiments the promoter is selected from the group consisting of aconstitutive promoter, an inducible promoter a developmentally regulatedpromoter and a tissue specific promoter.

According to still further features in the described preferredembodiments the long day plant overexpressing the phytochrome protein isa commercial dicotyledonous or monocotyledonous plant.

According to still further features in the described preferredembodiments the long day plant overexpressing the phytochrome protein isselected from the group consisting of an agronomic crop, anhorticultural crop, and an ornamental plant.

According to still further features in the described preferredembodiments the long day plant overexpressing the phytochrome protein isan annual or a perennial plant selected from the group consisting of arosette forming plant, a bulb forming plant, a corm forming plant, aherbaceous plant, a shrub forming plant and a tree forming plant.

According to still further features in the described preferredembodiments the flowering-shoots, flowering pots, flowers, seeds orfruits which develop under the substantially shorter days have at leastone improved agronomic and/or commercial characteristic selected fromthe group consisting of an increased number of flowering shoots, anincreased is number of flowers, an increased number of fruit formingflowers, a faster growth rate, a lower cold request for growth andflowering and reduced light intensity dependent flowering as compared tothe similar long day plant not overexpressing the phytochrome protein.

According to still further features in the described preferredembodiments the day length of the substantially shorter days is at least15% shorter than that required by the similar long day plant notoverexpressing the phytochrome protein.

According to still further features in the described preferredembodiments the substantially shorter days are effected by naturallighting conditions.

According to still further features in the described preferredembodiments the substantially shorter days are further characterized byat least one condition selected from the group consisting of a lightintensity between 80-2000 μmole m⁻²s⁻¹ PAR, and a temperature selectedfrom the range between 5-30° C.

According to still further features in the described preferredembodiments the long day plant is cultivated for commercial productionof flowering-shoots, flowering pots, flowers, seeds or fruits.

According to still further features in the described preferredembodiments the exogenous expression cassette for overexpressing thephytochrome protein is compatible for propagation in cells, orintegration into the genome, of a plant.

According to still further features in the described preferredembodiments the step of overexpressing the phytochrome protein in thelong day plant is effected by transforming the long day plant with anexpression cassette encoding a phytochrome protein.

According to still further features in the described preferredembodiments the phytochrome protein is phytochrome A or phytochrome B.

According to still further features in the described preferredembodiments modulating the responsiveness of the long day plant to daylength is utilized for producing flowering-shoots, flowering pots,flowers, seeds or fruits in the long day plant during substantiallyshorter days than those required by the long day plant for producing theflowering-shoots, flowering pots, flowers, seeds or fruits.

According to still further features in the described preferredembodiments modulating the responsiveness of the long day plant to daylength is utilized for causing a spring or summer flowering plant toflower during autumn, winter or year-round.

According to still further features in the described preferredembodiments modulating the responsiveness of the long day plant to daylength is utilized for conferring early flowering in the long day plantunder short-day conditions.

The present invention successfully addresses the shortcomings of thepresently known configurations by providing long day plantscharacterized by altered responsiveness to day length and methods ofgenerating same.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings. With specific reference now tothe drawings in detail, it is stressed that the particulars shown are byway of example and for purposes of illustrative discussion of thepreferred embodiments of the present invention only, and are presentedin the cause of providing what is believed to be the most useful andreadily understood description of the principles and conceptual aspectsof the invention. In this regard, no attempt is made to show structuraldetails of the invention in more detail than is necessary for afundamental understanding of the invention, the description taken withthe drawings making apparent to those skilled in the art how the severalforms of the invention may be embodied in practice.

In the drawings:

FIG. 1 illustrates photoperiod and sun-irradiance effects on growthcycle length in Aster “Sun Karlo” plants transformed with PHY A cDNA.Greehouse grown plants were exposed to 10 hours daylight or to naturalshort day extended to 16 hours light by incandescent lighting (0.5 μmolm−2 s⁻¹ PAR. The time needed for inflorescence shoot elongation andflowering was measured during two growth cycles under different naturalday conditions (Table 1). Grey and black columns indicate the first andsecond growth cycle, respectively.

FIG. 2 illustrates the time needed for inflorescence shoot elongation inAster, “Sun Karlo” plants and transformed PHY A lines AA20 and AA21 andPHY B lines AB13-1 and AB12-6 under natural-day extension withincandescent lighting in the greenhouse. Plants were grown in thegreenhouse under incandescent lighting (0.5 μmol m⁻² s⁻¹ PAR) at a daylength extended to 14 or 16 h. Time needed for inflorescence shootelongation to 55 cm in 90% of the plants was measured during two growthcycles under different natural day conditions (Table 1). Grey and blackcolumns indicate the first and second growth cycle, respectively.

FIG. 3 illustrates the time needed for inflorescence shoot elongation inAster, “Sun Karlo” plants and transformed PHY A lines AA20 and AA21 andPHY B lines AB13-1 and AB12-6 under the effect of natural day extensionwith fluorescent lighting. Plants were grown in the greenhouse underfluorescent lighting (0.5 μmol m⁻² s⁻¹ PAR) and extended day conditionsof 14 or 16 h. Time needed for inflorescence shoot elongation to 55 cmin 90% of the plants was measured during two growth cycles underdifferent natural day conditions (Table 1). Grey and black columnsindicate the first and second growth cycle, respectively.

FIG. 4 illustrates the time needed for inflorescence shoot elongation inAster, “Sun Karlo” plants and transformed PHY A lines AA20 and AA21 andPHY B lines AB13-1 and AB12-6, under natural-day conditions and nightbreak with incandescent or fluorescent lighting. Plants were grown undergreenhouse conditions and night-break treatments were applied at themiddle of the night for two hours with either fluorescent orincandescent lighting (0.5 μmole m⁻² S⁻¹ PAR). Time needed forinflorescence shoot elongation to 55 cm in 90% of the plants wasmeasured during two growth cycles under different natural day conditions(Table 1). Grey and black columns indicate the first and second growthcycle, respectively. FIG. 5 is photograph illustrating the effect of PHYA or PHY B overexpression on fruit development in transgenic Hypericumcv. “Excellent Flair” grown in a greenhouse under a gradual increase innatural day length (from February to June). At the end of this growthperiod, the transgenic lines (right side) exhibited enhanced floweringand fruit development, whereas the wild type plants (left side) werestill at the beginning of the flowering stage and needed an additionalgrowth period (one month) for fruit development.

FIGS. 6 a-b are photographs illustrating the effect of PHY A or PHY Boverexpression on inflorescent shoot development in transgenic Aster cv.“Sun Karlo” grown in a greenhouse under constant short days of 10 hoursexposure to sun irradiance. At the end of this growth period, thetransgenic lines overexpressing PHY A (FIGS. 6 a-b left plants) producedinflorescent shoots, which is a typical response of Aster to long days,whereas the PHY B (FIGS. 6 a-b middle plants) and the “Sun Karlo”control plants (FIGS. 6 a-b right plants) produced rosette shoots, atypical response of Aster to short days.

FIG. 7 illustrates flowering and fruit setting in transgenic Hypericumplants overexpressing either PHY A or PHY B and grown under fieldconditions and natural day extension to 14 h of light by low intensityincandescent lighting. Non-flowering wild type “Excellent Flair” plantsare on the left upper corner while flowering and fruit bearingtransgenic plants are on the right lower corner of the photograph.

FIGS. 8 a-g illustrate accumulated commercial yield of red-fruit bearingshoots harvested from various lines of Hypericum “Excellent Flair”plants overexpressing phytochrome under commercial field conditions.Commercial yields of transgenic plants (squares) overexpressing eitherPHY A (HA) or PHY B (HB) and the wild type cultivar “Excellent Flair”(EF, circles) were grown under natural day-length (solid line) ornatural day-length extended to 14 h of light (broken line).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of methods of generating long day plantscharacterized by altered responsiveness to day length and of plantsgenerated thereby which are of great commercial importance. Inparticular, the present invention can be utilized for generatingcommercial long day plants which can be cultivated for commercialproduction of flowering-shoots, flowering pots, flowers, seeds or fruitsunder short day conditions.

The principles and operation of the present invention may be betterunderstood with reference to the accompanying descriptions.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description. The invention is capable of otherembodiments or of being practiced or carried out in various ways. Also,it is to be understood that the phraseology and terminology employedherein is for the purpose of description and should not be regarded aslimiting.

As used herein, the phrase “long day plant” refers to a plant, whichinitiates or accelerates the initiation of its flower formationfollowing exposure to a day length longer than a critical day length.The critical day length is a specific feature of each plant but everyplant which responds (in flowering) to days longer than its critical daylength is considered a “long day plant”. Examples of long day plantsinclude, but are not limited to, beet, radish, lettuce, spinach,Arabidopsis, Antirrhinum, Avena sativa, Pisum sativum, Hordeum vulgare,chrysanthemum, Brassica, Campanula, Delphinium, Dianthus, Fuchia,Gypsophilia, Helichrysum, Hyoscyamus, Jasminium, Lolium, Lunaria,Nicotiama sylvestris, Phlox, Salvia, Petunia, Trachelium, Trifolium,Triticum aestivum, Vicia faba and Sinapis alba (For additional examples,see Thomas and Vince-Prue, 1997).

As used herein, the phrase “shorter days” refer to days which include alighting period which is substantially shorter than the critical daylength required by a long day plant to flower. As is further describedin the Examples section which follows, the lighting period of shorterdays is 10-15% or more, shorter than that required for flowering in longday plants.

As used herein, the phrase “natural lighting conditions” refers tolighting which is identical to sunlight in its spectral components,intensity range, temperature to light-intensity relationship and/orseasonal dependence.

As used herein, the phrase “overexpressing a phytochrome protein”typically refers to generating phytochrome protein activity in some orall of the cells of a long day plant, which is substantially higher thanthe activity of this protein in a wild type plant grown under the sameconditions. Typically, such over expression is effected by increasingthe phytochrome concentration in the cell via stable or transienttransformation with exogenous sequence(s), although upregulation ofendogenous gene expression via “gene knock-in” of upregulatory sequencesupstream to an endogenous phytochrome coding sequence is also envisaged.

The ability to control plant flowering and fruit development incommercially cultivated plants is of great importance to a grower. Suchcontrol can be exercised to increase flower and fruit yield, to growcrops under previously unsuitable conditions or to advance, delay orextend a growing season. As such, various prior art studies haveattempted to generate plants characterized by early flowering. Thesestudies have enjoyed limited success since such flowering depended uponconditions, which could not be reproduced in the field.

While reducing the present invention to practice, the present inventorshave succeeded in generating long day plants which flower under shortday conditions even when such conditions included commercial lightingconditions.

As used herein, the phrase “commercial lighting conditions” refers togrowth conditions that include exposure to natural lighting conditionswith or without addition of artificial lighting.

Thus according to one aspect of the present invention, there is provideda method of modulating a responsiveness of a long day plant to daylength conditions, in particular modulating the responsiveness of longday plants to short day conditions which includes sun irradiance.

Preferably, shorter days are characterized by a lighting period shorterthan a critical length, as well as specific spectral, light intensityand temperature conditions, since in a long day plant, light intensityand temperature conditions affect the critical day length, which becomesshorter with an increase in light intensity and/or temperature.

The method is effected by overexpressing a phytochrome protein in atleast a portion of the cells of the long day plant under conditions suchthat flowering-shoots, flowering pots, flowers, seeds or fruits of thelong day plant develop under substantially shorter days than thoserequired for development, or for accelerated development, of theflowering-shoots, flowering pots, flowers, seeds or fruits in a similarlong day plant not over expressing the phytochrome protein.

According to a preferred embodiment of the present invention, the stepof overexpressing the phytochrome protein in the long day plant iseffected by transforming at least a portion of the cells of the long dayplant with an expression cassette including a phytochrome A orphytochrome B coding sequence being under the transcriptional control ofa plant functional promoter. Any phytochrome A or phytochrome B encodingsequences can be utilized by the present invention including sequencesderived from Oat, Cucurbita, Pea, Maize, Arabidopsis, Rice, Potato,Tobacco and any other non-angiosperm plant (see, for example Mathews andSharrock, 1997 for additional details).

The plant functional promoter can be, for example, a constitutivepromoter, such as for example, the Cauliflower Mosaic virus (CaMV) 35Spromoter or the Ubiquitin promoter; an inducible promoter such as thetetracycline inducible promoter; or a developmentally regulated ortissue specific promoter.

Specific examples of suitable expression cassettes and expressionconstruct harboring such cassettes are given in the Examples section,which follows.

Plant transformation using the phytochrome A or B expression cassettesdescribed herein can be effected via any method known in the art forintroducing nucleic acid constructs into both monocotyledonous anddicotyledonous plants (Potrykus, I., Annu. Rev. Plant. Physiol., Plant.Mol. Biol. (1991) 42:205-225; Shimamoto et al., Nature (1989)338:274-276). Such methods rely on either stable integration of thenucleic acid construct or a portion thereof into the genome of theplant, or on transient expression of the nucleic acid construct in whichcase these sequences are not inherited by a progeny of the plant.

There are two principle methods of effecting stable genomic integrationof exogenous sequences such as those included within the nucleic acidconstructs of the present invention into plant genomes:

(i) Agrobacterium-mediated gene transfer: Klee et al. (1987) Annu. Rev.Plant Physiol. 38:467-486; Klee and Rogers in Cell Culture and SomaticCell Genetics of Plants, Vol. 6, Molecular Biology of Plant NuclearGenes, eds. Schell, J., and Vasil, L. K., Academic Publishers, SanDiego, Calif. (1989) p. 2-25; Gatenby, in Plant Biotechnology, eds.Kung, S. and Arntzen, C. J., Butterworth Publishers, Boston, Mass.(1989) p. 93-112.

(ii) direct DNA uptake: Paszkowski et al., in Cell Culture and SomaticCell Genetics of Plants, Vol. 6, Molecular Biology of Plant NuclearGenes eds. Schell, J., and Vasil, L. K., Academic Publishers, San Diego,Calif. (1989) p. 52-68; including methods for direct uptake of DNA intoprotoplasts, Toriyama, K. et al. (1988) Bio/Technology 6:1072-1074. DNAuptake induced by brief electric shock of plant cells: Zhang et al.Plant Cell Rep. (1988) 7:379-384. Fromm et al. Nature (1986)319:791-793. DNA injection into plant cells or tissues by particlebombardment, Klein et al. Bio/Technology (1988) 6:559-563; McCabe et al.Bio/Technology (1988) 6:923-926; Sanford, Physiol. Plant. (1990)79:206-209; by the use of micropipette systems: Neuhaus et al., Theor.Appl. Genet. (1987) 75:30-36; Neuhaus and Spangenberg, Physiol. Plant.(1990) 79:213-217; or by the direct incubation of DNA with germinatingpollen, DeWet et al. in Experimental Manipulation of Ovule Tissue, eds.Chapman, G. P. and Mantell, S. H. and Daniels, W. Longman, London,(1985) p. 197-209; and Ohta, Proc. Natl. Acad. Sci. USA (1986)83:715-719.

Following transformation plant propagation is exercised. Regenerationcan be effected by seed propagation or vegetative propagation methods,which are well known in the art.

In addition to stable genomic expression, the phytochrome A or Bexpression cassettes can also be transiently expressed in a whole plantor in specific tissue regions thereof, including, for example, the shootapical meristem (SAM) or leaves. Thus, in this case, transienttransformation methods are utilized for transiently expressing thephytochrome A or B expression cassettes. Such methods include, but arenot limited to, microinjection and bombardment as described above butunder conditions which favor transient expression. For example,biolistic bombardment of shoot apical meristems can be utilized totransiently express phytochrome A or B therein.

In addition, packaged or unpackaged recombinant virus vector includingthe phytochrome A or B expression cassette can be utilized to infectplant tissues or cells such that a propagating recombinant virusestablished therein expresses phytochrome A or B either in a tissuerestricted manner or in the entire plant (systemic infection).

Viruses that have been shown to be useful for the transformation ofplant hosts include CaMV, TMV and BV. Transformation of plants usingplant viruses is described in U.S. Pat. No. 4,855,237 (BGV), EP-A 67,553(TMV), Japanese Published Application No. 63-14693 (TMV), EPA 194,809(BV), EPA 278,667 (BV); and Gluzman, Y. et al., Communications inMolecular Biology: Viral Vectors, Cold Spring Harbor Laboratory, NewYork, pp. 172-189 (1988). Pseudovirus particles for use in expressingforeign DNA in many hosts, including plants, is described in WO87/06261.

Construction of plant RNA viruses for the introduction and expression ofnon-viral exogenous nucleic acid sequences in plants is demonstrated bythe above references as well as by Dawson, W. O. et al, Virology (1989)172:285-292; Takamatsu et al. EMBO J. (1987) 6:307-311; French et al.Science (1986) 231:1294-1297; and Takamatsu et al. FEBS Letters (1990)269:73-76.

According to another preferred embodiment of the present invention, thestep of overexpressing the phytochrome protein in the long day plant iseffected by “gene knock-in” of an exogenous polynucleotide encoding atranscriptional or translational enhancer.

Thus, for example, gene knock-in constructs including sequenceshomologous with regions upstream or downstream of the endogenous PHY Aor B sequences can be generated and used to position transcriptional ortranslational enhancer sequence in cis regulatory control of theendogenous PHY A or B sequences to thereby upregulate expression of thisgene.

These constructs preferably include positive and negative selectionmarkers and may therefore be employed for selecting homologousrecombination events. One ordinarily skilled in the art can readilydesign a knock-in construct including both positive and negativeselection genes for efficiently selecting transformed plant cells thatunderwent a homologous recombination event with the construct. Suchcells can then be grown into full plants. Standard methods known in theart can be used for implementing knock-in procedures. Such methods areset forth in, for example, U.S. Pat. Nos. 5,487,992, 5,464,764,5,387,742, 5,360,735, 5,347,075, 5,298,422, 5,288,846, 5,221,778,5,175,385, 5,175,384, 5,175,383, 4,736,866 as well as Burke and Olson,Methods in Enzymology, 194:251-270, 1991; Capecchi, Science244:1288-1292, 1989; Davies et al., Nucleic Acids Research, 20 (11)2693-2698, 1992; Dickinson et al., Human Molecular Genetics,2(8):1299-1302, 1993; Duff and Lincoln, “Insertion of a pathogenicmutation into a yeast artificial chromosome containing the human APPgene and expression in ES cells”, Research Advances in Alzheimer'sDisease and Related Disorders, 1995; Huxley et al., Genomics, 9:742-7501991; Jakobovits et al., Nature, 362:255-261 1993; Lamb et al., NatureGenetics, 5: 22-29, 1993; Pearson and Choi, Proc. Natl. Acad. Sci. USA,1993, 90:10578-82; Rothstein, Methods in Enzymology, 194:281-301, 1991;Schedl et al., Nature, 362: 258-261, 1993; Strauss et al., Science,259:1904-1907, 1993, WO 94/23049, WO93/14200, WO 94/06908 and WO94/28123 also provide information.

Thus, the method of the present invention can be utilized to modulateday length responsiveness in commercial dicotyledonous ormonocotyledonous plants including, but not limited to, agronomic cropplants, horticultural crop plants, or ornamental plants. Examples ofsuch plants include, but are not limited to, rosette forming plants suchas Crysantheumum, Solidago, Solidaster, Gypsophyla, TracheliumHyoscyamus, Lunaria and Scabiosa; bulb forming plants such as Allium,Lilium and Alstromeria; corm forming plants such as Aconitum, Anemone,Ranunculus, Liatris and Asclepias tuberosa; Herbaceous plants such asAnagallis, Campanula, Nigella, and Phlox; or Shrubs such as Fuchia,Hibiscus and Jasminium.

As is further described in the Examples section below, the plantsgenerated according to the teachings of the present invention respond,in flowering, to short day conditions even when such conditions areprovided by natural lighting utilized in commercial cultivation. Thus,unlike prior art methods, the method of the present invention is highlysuitable for commercial applications since most commercial crops aregrown in soil under such lighting conditions.

As is further described in the examples section which follows, thephytochrome overexpressing long day plant of the present invention ischaracterized by at least one improved agronomic and/or commercialcharacteristic including, but not limited to an increased number offlowering shoots, an increase number of flowers, an increased number offruit forming flowers, a faster growth rate, a lower cold request forgrowth and flowering and a reduced light intensity dependent inductionor acceleration of flowering as compared to the similar long day plantnot overexpressing the phytochrome protein.

For example, a transgenic Hypericum cv. “Excellent Flair” plantgenerated according to the teachings of the present invention and grownunder natural conditions over a growth period which covered winterthrough spring (in which natural day length is gradually elongated),reached the fruit development stage over a month before a similarnon-transgenic plant (see the Examples section for further detail).

Transgenic Aster cv “Sun Karlo” plants generated according to theteaching of the present invention produced flowering shoots ofcommercial value under natural short days of winter in commercialgreenhouse (see the Examples section below for further detail).

It will be appreciated that the above described characteristics of thetransgenic long day plant of the present invention is of tremendouscommercial value, since it enables the use of such a plant in commercialproduction of flowering-shoots, flowering pots, flowers, seeds or fruitsunder light conditions not suitable for such production from a wild typelong day plant.

Thus, the method of the present invention would enable a grower toprolong a particular growing season of specific long day plants therebygenerating higher yields from crops. In addition, growers at differentparts of the world will be able to adopt new cultivars, which werepreviously restricted to specific geographical regions.

For example, plants which require a day length of over 13 hours forflowering will not flower under natural conditions in an equatorial zoneunless their critical day length is shortened.

Further more, the method of the present invention will enable a growerto start cultivating a commercial crop earlier in the season or latertoward the end of the season (depending on season), thereby enabling thegrower to reach the marketplace earlier or to extend the market seasonof a particular plant product.

As is further described in the Examples section which follows, theresponse to day length varied between phytochrome A and phytochrome Boverexpressing plants under the various lighting conditions tested.These variations enable the overexpression of a specific phytochrome fora specific purpose. For example, PHY A over expression can be utilizedfor year-round flowering, while PHY B overexpression can be utilized forextended flowering period and increased number of flowering shoots.

Thus, it will be appreciated that selective co-expression of bothphytochromes in a long day plant can possibly be utilized to furtherenhance the plants response to various day length conditions.

For example, long day plants which overexpress both phytochrome A and Bcan be generated using double transformation techniques or by sexuallycrossing phytochrome A and phytochrome B expressing plants of the samecultivar. The phytochrome A and B encoding sequences can each be placedunder the transcriptional control of a different induced promoterthereby enabling selective expression of one or both at any time duringgrowth.

Additional objects, advantages, and novel features of the presentinvention will become apparent to one ordinarily skilled in the art uponexamination of the following examples, which are not intended to belimiting. Additionally, each of the various embodiments and aspects ofthe present invention as delineated hereinabove and as claimed in theclaims section below finds experimental support in the followingexamples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions, illustrate the invention in a non-limiting fashion.

Generally, the nomenclature used herein and the laboratory proceduresutilized in the present invention include molecular, biochemical,microbiological and recombinant DNA techniques. Such techniques arethoroughly explained in the literature. See, for example, “MolecularCloning: A laboratory Manual” Sambrook et al., (1989); “CurrentProtocols in Molecular Biology” Volumes I-III Ausubel, R. M., ed.(1994); Ausubel et al., “Current Protocols in Molecular Biology”, JohnWiley and Sons, Baltimore, Maryland (1989); Perbal, “A Practical Guideto Molecular Cloning”, John Wiley & Sons, New York (1988); Watson etal., “Recombinant DNA”, Scientific American Books, New York; Birren etal. (eds) “Genome Analysis: A Laboratory Manual Series”, Vols. 1-4, ColdSpring Harbor Laboratory Press, New York (1998); methodologies as setforth in U.S. Pat. Nos. 4,666,828; 4,683,202; 4,801,531; 5,192,659 and5,272,057; “Cell Biology: A Laboratory Handbook”, Volumes I-III Cellis,J. E., ed. (1994); “Culture of Animal Cells—A Manual of Basic Technique”by Freshney, Wiley-Liss, N.Y. (1994), Third Edition; “Current Protocolsin Immunology” Volumes I-III Coligan J. E., ed. (1994); Stites et al.(eds), “Basic and Clinical Immunology” (8th Edition), Appleton & Lange,Norwalk, Conn. (1994); Mishell and Shiigi (eds), “Selected Methods inCellular Immunology”, W. H. Freeman and Co., New York (1980); availableimmunoassays are extensively described in the patent and scientificliterature, see, for example, U.S. Pat. Nos. 3,791,932; 3,839,153;3,850,752; 3,850,578; 3,853,987; 3,867,517; 3,879,262; 3,901,654;3,935,074; 3,984,533; 3,996,345; 4,034,074; 4,098,876; 4,879,219;5,011,771 and 5,281,521; “Oligonucleotide Synthesis” Gait, M. J., ed.(1984); “Nucleic Acid Hybridization” Hames, B. D., and Higgins S. J.,eds. (1985); “Transcription and Translation” Hames, B. D., and HigginsS. J., eds. (1984); “Animal Cell Culture” Freshney, R. I., ed. (1986);“Immobilized Cells and Enzymes” IRL Press, (1986); “A Practical Guide toMolecular Cloning” Perbal, B., (1984) and “Methods in Enzymology” Vol.1-317, Academic Press; “PCR Protocols: A Guide To Methods AndApplications”, Academic Press, San Diego, Calif. (1990); Marshak et al.,“Strategies for Protein Purification and Characterization—A LaboratoryCourse Manual” CSHL Press (1996); all of which are incorporated byreference as if fully set forth herein. Other general references areprovided throughout this document. The procedures therein are believedto be well known in the art and are provided for the convenience of thereader. All the information contained therein is incorporated herein byreference.

Example 1 Generation of Agrobacterium Tumefaciens Harboring PhytochromeA or B Expression Cassettes

Phytochrome A Expression Cassette:

An Oat (Avena sativa) phytochrome A polynucleotide fragment whichincludes a “type 5” (GenBank Accession Number X03244) cDNA and “type 3”(GenBank Accession Number X03242) cDNA was provided in pFY122 (Boylan,M. T. and Quail, P. H., 1989, Plant Cell, 1: 765-773).

The oat phytochrome A polynucleotide fragment was excised from pFY122and subcloned into the plasmid pROK2 generating the pRFYI plasmid vector(Smith Harry U.S. Pat. No. 5,945,579). The resultant pRFYI constructincluded the PHY A sequence subcloned downstream to the CaMV 35Spromoter, and upstream to the polyadenylation signal sequences derivedfrom the CaMV 35S transcript. The construct also included a bacterialselection marker for positive bacterial selection and a plantkanamycin-resistance coding sequence for positive plantlets selection(Beyan, M., 1984, Nucleic Acid Research, 12: 8711).

This pRFYI construct was used to transform Agrobacterium tumefaciensstrain 2260 (Smith Harry U.S. Pat. No. 5,945,579) which was utilized forplant transformation in order to generate transgenic plantsoverexpressing phytochrome A.

Phytochrome B Expression Cassette:

An Arabidopsis phytochrome B (PHY B) coding sequence (GenBank Accessionnumber X17342, Sharrock, R. A. and Quail, P. H. 1989. Genes Dev. 3,1745-1757) was PCR amplified from a λEMBL3 clone (Somer, D. E.,Sharrock, R. A., Tepperman, J. M. and Quail, P. H. 1991. The Plant Cell,3, 1263-1274) and sequenced.

The PHY B PCR product was subcloned into pROK2 vector (Beyan, M. 1984.Nucl. Acids Res. 12, 8711-8721) to generate a pROKB plant expressionvector in which PHY B is positioned in a sense orientation downstream toa CaMV 35S promoter and upstream to a polyadenylation signal. The pROKBconstruct further included a bacterial selection marker for positivebacterial selection and a plant kanamycin-resistance coding sequence forpositive plantlets selection.

The pROKB construct was used to transform Agrobacterium tumefaciensstrain LBA4404 (Halliday, K. J., Thomas, B. and Whitelam G. C., 1997;The plant Journal 12, 1079-1090) having a chromosomally locatedrifampicin resistance gene. The transformed Agrobacterium was utilizedfor plant transformation in order to generate transgenic plantsoverexpressing phytochrome B.

Example 2

Generation and Analysis of Transgenic Aster cv. “Sun Karlo”

Aster plants were infected with Agrobacterium carrying expressioncassettes for either phytochrome A or B in order to generate commerciallong day plant characterized by an altered flowering pattern under shortday conditions.

Plant Material:

Aster plants of cultivar “Sun Karlo” were grown in a controlledenvironment (phytotrone) in which sunlight was used as light sourceduring at least a part of the photoperiod. In order to keep the plantsin their rosette vegetative growth stage, the growing conditions were asfollows:

-   -   (1) short-day lightning of 10 hours exposure to sun irradiance    -   (2) controlled temperatures of 20:12° C. during day:night        periods respectively.

Plant Transformation:

Leaves of rosette shoots were surface sterilized in a 70% (v/v) ethanolfor 1 minute followed by 10% (v/v) solution of domestic bleach for 8minutes. Phytochrome carrying constructs were mobilized into the plantgenome via Agrobacterium tumefacience infection. Leaf discs were excisedand soaked in a 1/20 dilution of an overnight culture of theAgrobacterium strain containing the oat- PHY A-cDNA vector (pRFYI) orthe Arabidopsis-PHY B-cDNA vector (PROKB) separately. Infected leafdiscs were placed onto MS-salts medium plates containing 20 g/l sucrose,0.1 mg/l naphthaleneacetic acid, 1.0 mg/l 6-benzylaminopurine and 7 g/lagar. Plates were incubated under constant temperature of 20° C. and24-hour cycles of 16 hours low light intensity from cool whitefluorescent lamps followed by eight hours of dark. Two days later, leafdiscs were transferred onto fresh MS-salts medium plates containing 20g/l sucrose, 0.1 mg/l naphthaleneacetic acid, 1.0 mg/l6-benzylaminopurine and 7g/l agar with the addition of 100 mg/lkanamycin and 400 mg/l augmentin for selection.

Leaf discs were transferred onto similar fresh medium every two weeksuntil regenerated shoots were observed. The putative transgenic shootswere excised and transferred to MS-salts medium (MS salts supplementedwith 20g/l sucrose, 100 mg/l kanamycin, 400 mg/l augmentin and 7g/1agar). This medium was replaced every 2 weeks until roots developed.

The formation of roots by the excised shoots in the presence ofkanamycin was an indication that the plants have been successfullytransformed with the desired expression vector. Small plantlets (Shootswhich developed roots) of approximately 1.0 cm to 2.0 cm in length wereremoved from the media and transplanted in soil containing pots forhardening in the phytotrone controlled environment. The selectedcontrolled environment included: high humidity, short-day of 10 hourssun irradiance and controlled temperatures of 20:12 ° C., day:night,respectively.

Selection of Positive Transformed Aster Plants:

Leaves of hardened plants were PCR analyzed using forward primer5′-CGCCTTCTGGCTATCAGATG -3′ (SEQ ID NO: 1) and reverse primer5′-CGAGGAAGCATTGCTACTGT-3′ (SEQ ID NO: 2) specific to the Oat PHY Asequence, and forward primer 5′-GCTTGTTCCAGCAAGGACTACT-3′ (SEQ ID NO: 3)and reverse primer 5′-GCTGCTATGGAACATGTCT-3′ (SEQ ID NO: 4) specific tothe Arabidopsis PHY B coding sequence.

PCR positive plants were further analyzed for expression of theexogenous cDNAs. This was performed by employing reversetranscription-PCR (RT-PCR) analysis on mRNA derived from the transformedplants. For the reverse-transcription step, a 12-18 mer oligo (dT)primer (GibcoBRL Cat #. 18418-012) was used in the presence of thereverse-transcriptase superscript II (Gibco-BRL Cat #18064-022). Primersspecific to phytochrome cDNA (SEQ ID NOs:1 and 2 or 3 and 4) were usedin the PCR amplification step following the RT step.

Positive selected plants were vegetatively propagated. Cuttings ofrosette shoots were rooted under phytotrone controlled environment whichincluded short-days of 10 hour sun irradiance and temperatures of 20:12°C. day:night, respectively.

Three generations of plants of each transformant were grown prior toconducting physiological experiments designed for assessing the responseof the Aster transgenic plants to day-length.

Cultivation Conditions:

Day-length treatments under phytotrone or greenhouse conditions wereapplied to transgenic Aster (cv “Sun Karlo”) carrying PHY A or PHY Bcoding sequences.

Phytotrone experiment: phytotrone conditions were as follows: acontrolled temperature of 20:12° C., day:night respectively; lightningconditions of 8 hour exposure to sun irradiance (400 μmolm⁻² s⁻¹ PAR)followed by day extension treatments of 2 hours which were appliedeither with sun irradiance or with artificial light from incandescentplus fluorescent lamps. The artificial light intensities were asfollows: 20 μmol m⁻²s⁻¹ PAR, 8.1 μmol m⁻²s⁻¹ in the red (600-700 ηm) and8.0^(˜) μmol m⁻²s⁻¹ in the far-red (700-800 ηm) spectra with R:FR ratioof 1.02, similar to that of sun irradiance. Night break treatmentsincluded short lighting, from incandescent or fluorescent lamps, for 15or 30 minutes which were introduced in the middle of the 16 hours darkperiod. The incandescent lamp light intensity was 1.4 μmol m⁻²s⁻¹ PAR,0.9 μmol m⁻² s⁻¹ red (600-700 ηm) and 1.5 μmol m⁻²s⁻¹ far-red (700-800ηm) with R:FR ratio of 0.6. Fluorescent lamp light intensity was 2.4μmole m⁻²s⁻¹ PAR, 1.1 μmole m⁻² s⁻red (600-700 μm) and 0.1 5 μmolem⁻²s⁻¹ far-red (700-800 ηm), with R:FR ratio of 7.3.

Greenhouse experiment: the greenhouse experiment started at September,and included two growth cycles, which ended in April.

Growth conditions were as follows: a temperature of 25:14° C., day:night, respectively. Natural short days and sunlight conditions wereused as the basis for day extension and night break treatments withartificial lighting. Several different treatment were applied asfollows:

-   -   (i) a 10 hours exposure to sun irradiance was applied constantly        during the entire growth cycle; or    -   (ii) each growth cycle started with long-day conditions during        inflorescent-shoot elongation followed by natural day conditions        during flower development.

Long-day conditions included day extension or night break treatments,which were applied with incandescent or fluorescent lamps having a lightintensity of 0.5 μmole m⁻²s⁻¹ PAR. Natural day lightning was extended tolight-period (photoperiod) of 14 or 16 hours. Alternatively, natural daylightning was followed by a night break of 2 hours. The long daytreatments were applied during inflorescent shoot elongation until ashoot length of 50-55 cm was reached. When 90% of the shoots reachedthis length the plants were transferred to natural day length conditionsfor flower initiation and flowering. At flowering, shoots were cut backto soil surface and the plants were transferred back to long dayconditions for the second growth cycle which started from rosetteshoots.

Example 3 Response of the Transgenic Aster to Various Day-LengthTreatments

Critical Day Length in Transgenic Aster Plants:

Table 1 below represents the effect of day extension treatments in thephytotrone on the transition from rosette to inflorescent shootdevelopment and flowering. As mentioned above the plants were exposed tosun irradiance for eight hours. Two hours of day extension were appliedeither by additional sun irradiance (high light intensity) or artificiallight from incandescent and fluorescent lamps (low light intensity).

According to the data in Table 1 and as shown in FIG. 6, overexpressionof phytochrome A considerably shortened the critical day length andenabled inflorescent shoot development and flowering in the transgenicline AA4-7 to occur under day length of eight hours sun irradiance. Daylength of 10 hours was short for non-transgenic plants (“Sun Karlo”) andtherefore their inflorescent shoots did elongate and as a consequencedid not flower. Similarly, day length of 10 hours was short for plantsoverexpressing phytochrome B (data not shown).

Another PHY A overexpressing transgenic line: AA17-3 had critical daylength of 10 hours. Its inflorescent shoots did not develop under daylength of eight hours, but when the day length was extended from eighthours to ten hours, transgenic line AA17-3 developed inflorescentshoots. The difference in critical day length between the two PHY Aoverexpressing lines and between the transgenic and non-transgenic linesindicates that overexpression of phytochrome A does not eliminate therequirement for long days but shortens the critical day length neededfor a long day effect. As a consequence, days shorter than an originalcritical day length are perceived as long days by the transgenic plantsof the present invention.

The number of rosette leaves produced prior to shoot elongationindicated the physiological time of transition from rosette toinflorescent-shoot elongation (the beginning of the flowering process),a typical response of Aster to long day conditions. The number ofrosette leaves produced on transgenic and non-transgenic plants underthe day extension treatments further indicated that PHY A overexpressiondid not eliminate the effect of day length and light intensity on thetransition process but enabled its operation under a substantiallyshorter day length and lower light intensity. While “Sun Karlo, thenon-transgenic line, continuously produced rosette leaves under eight orten hours of light independent of light intensity during day extension,the PHY A overexpressing lines: AA4-7 and AA17-3 accelerated thetransition process (produced less rosette leaves prior to thetransition) as a response to an increase in day length and/or lightintensity.

Between the two PHY A lines AA4-7 was less sensitive to the photoperiodtreatments and to the difference between day extension with artificiallighting and that with sun irradiance indicating lower sensitivity ofthe PHY A overexpressing plants to light intensities. TABLE 1 Day-lengthand sun-irradiance effects on inflorescent shoot and flower developmentin Aster “Sun Karlo” plants transformed with PHY A cDNA. Plants wereexposed to eight hours sun irradiance extended to ten hours by sun orartificial lighting. Time to Rosette leaves at Transgenic Light sourcefor day flowering flowering line extension Days No. AA 4-7 Non 72 ± 0.86.8 ± 0.5 AA 4-7 Lamps 76 ± 2.0 5.2 ± 0.4 AA 4-7 Sun 77 ± 0.4 4.8 ± 0.4AA 17-3 Non 12.2 ± 1.0  AA 17-3 Lamps 88 ± 3.7 6.5 ± 0.7 AA 17-3 Sun 95± 4.5 3.8 ± 0.5 Sun Karlo Non  15 ± 1.2 Sun Karlo Lamps 14.8 ± 1.8  SunKarlo Sun 15.7 ± 0.9 

Night Break Effect on Transgenic and “Sun Karlo” Aster Plants

Incandescent Lighting:

Light applied in the middle of the night (night break) has a long dayeffect on flowering in long day plants. To affect flowering in long dayplants the length of the night break should exceed a minimum length.Table 2 below represent the effect of an eight hour exposure to sunirradiance (short day) followed by 15 or 30 minutes of night break withincandescent lighting on inflorescent shoot development in transgenicplants overexpressing phytochrome A or B. Light from incandescent lampincludes both red and far-red spectrum but is relatively rich in thefar-red spectrum.

Night break treatments (15 or 30 minutes) were too short for thenon-transgenic “Sun Karlo” plants and therefore they did not developinflorescent shoots but continued to produce rosette leaves. This nightbreak duration, induced inflorescent shoot development and flowering intransgenic lines overexpressing either PHY B: AB12-6 and AB3-1 or PHY A:AA17-3. The PHY A line AA4-7, which developed inflorescent shoot undereight hours exposure to sun irradiance (see Table 1), accelerated thetransition from rosette to inflorescent shoot (produced less rosetteleaves) in response to this night break treatment.

The effect of night break on the PHY B lines: AB12-6 and AB 3-1 washigher than the effect of day extension and therefore plants that didnot develop inflorescent shoots under day length of 10 hours (Table 1)did so under 15 minutes night break (Table 2). Overexpression of eitherphytochrome A or B did not eliminate the plants response to the nightbreak duration as indicated by the lower number of rosette leaves theyproduced under the longer night break. However, these results clearlydemonstrate that phytochrome A or B overexpressing plants respond tolimited long day treatments (short night breaks), which have no effecton non-transformed plants. TABLE 2 Inflorescent-shoot and flowerdevelopment on Aster “Sun Karlo” plants transformed with Oat-PHY A orArabidopsis-PHY B. Plants were treated with eight hours exposure to sunirradiance followed by short night break with incandescent lighting inthe phytotrone. Rosette Time to leaves at Transgenic Night breakduration flowering flowering lines Min. Days No. AA 4-7 15 76 ± 1.0 5.0± 0.4 AA 4-.7 30 77 ± 1.5 3.8 ± 0.2 AA 17-3 15 — 10.7 ± 1.1  AA 17-3 3098 ± 2.0 5.5 ± 0.6 AB 12-6 15 99 ± 4.6 8.6 ± 0.8 AB 12-6 30 99 ± 2.6 5.5± 0.6 AB 3-1 15 101 ± 1.0  4.8 ± 0.8 AB 3-1 30 100 ± 2.6  4.8 ± 0.2 “Sunkarlo” 15 — 12.3 ± 0.9  “Sun karlo” 30 — 13.0 ± 2.1 

Fluorescent Lighting:

The night break lighting conditions described above was also providedfrom fluorescent lamps, which produce the red but not the far-redspectrum.

As is shown in Table 3, elimination of far-red light did not change theresponse of “Sun Karlo' plants to the short night break and thereforethey continued to produce rosette leaves and did not flower. The PHY Boverexpressing lines: AB12-6 and AB3-1 responded to the short nightbreak treatments with fluorescent lighting in inflorescent shootdevelopment and flowering and to the increased duration of lighting withreduced number of rosette leaves. The PHY A overexpressing line: AA17-3,which responded to 30 minutes of night break with incandescent light ininflorescent shoot development and flowering, did not respond to similarnight break duration when applied via fluorescent lighting. Thisspecific response to far-red rich light is typical of active phytochromeA indicating stable expression of PHY A cDNA in the transgenic plants ofthe present invention.

A similar conclusion can by drawn for the PHY B transgenic lines, whichresponded to night break treatments provided by both incandescent orfluorescent lamps. TABLE 3 Inflorescent-shoot and flowers development onAster “Sun Karlo” plants transformed with oat-PHY A or Arabidopsis-PHYB. Plants were treated with eight hours exposure to sun irradiancefollowed by short night break with fluorescent lighting in thephytotrone. Night break Rosette Transgenic lines duration Time toflowering leaves at flowering ID Min. Days No. AA 4-7 15 80 ± 1.3 6.4 ±0.6 AA 4-.7 30 67 ± 1.6 4.7 ± 0.2 AA 17-3 15 — 12.8 ± 0.4  AA 17-3 30 —13.9 ± 0.4  AB 12-6 15 82 ± 2.6 6.5 ± 0.5 AB 12-6 30 64 ± 1.6 4.9 ± 0.5AB 3-1 15 77 ± 1.7 7.4 ± 0.8 AB 3-1 30 57 ± 2.1 6.0 ± 0.4 “Sun karlo” 15— 15.5 ± 0.7  “Sun karlo” 30 — 16.8 ± 1.2 

Example 4 Development of Inflorescent Shoots in Transgenic Aster PlantsUnder the Influence of Natural Sun Irradiance Conditions in theGreenhouse

As mentioned above, the intensity of sunlight affects the critical daylength in some long day plants including Aster. To test the effect ofsun irradiance on the rate of inflorescent shoot development,phytochrome overexpressing “Sun Karlo” plants were grown for two cyclesin a greenhouse under a constant photoperiod but with varying sunirradiance conditions. The natural day length was shorten to constantphotoperiod of 10 hours or extended to 14 or 16 hours. The natural sunirradiance conditions during inflorescent shoot development in twogrowth cycles are presented in Table 4 below. As can be seen therein, inthe first growth cycle, the natural day length was longer, and the sunirradiance was higher than in the second growth cycle. TABLE 4 Naturalday length and sun irradiance conditions during the period ofinflorescent shoot development in two growth cycles of transgenic andnon-transgenic Aster cv. “sun Karlo” in the greenhouse. Day-lengthSun-irradiance h:min μmole m⁻²s⁻¹ First growth cycle 12:00-10:401000-500  Second growth cycle 10:06-11:00 300-500

Natural Day Extension with Incandescent Lighting:

As shown in FIG. 1, the non-transgenic “Sun Karlo” plants were highlyresponsive to photoperiod and sun irradiance conditions provided in thegreenhouse. These plants did not develop inflorescent shoots underphotoperiod of 10 hours, independent of sun irradiance intensity.However under photoperiod of 16 hours their first growth cycle wasconsiderably shorter than the second one.

In the greenhouse (FIG. 1) as well as under the phytotrone conditionsoverexpression of PHY A shortened the critical day length, therefore thePHY A overexpressing lines: AA17-3, AA2-8, AA20, AA21 and AA4-7developed inflorescent shoots and flowers during the two growth cyclesunder constant day length of 10 hours.

Under the 10 hour photoperiod the effect of sunlight intensity on thegrowth cycle length (the difference between the length of the first andthe second growth cycle) differed among the PHY A overexpressing lines:AA17-3, AA2-8, AA20, AA21 and AA4-7. Overexpression of phytochrome Aalmost overcame the effect of sunlight intensity on shoot development inthe line AA4-7 and its growth cycles were nearly unaffected by sunirradiance conditions as well as photoperiod.

Under a photoperiod of 16 hours the difference between the length of thetwo growth cycles was higher in “Sun Karlo” than in the four PHY Atransgenic lines AA2-8, AA20, AA21 and AA4-7. These PHY A overexpressinglines will produce flowering shoots not only under substantial shortdays but also under low sun irradiance, thus enabling flowering underwinter conditions.

FIG. 2 describes the time needed for transgenic and “Sun Karlo” plantsto develop 50 cm length of inflorescent shoots under photoperiods of 14and 16 hours, based on natural day extension with incandescent lighting.

“Sun Karlo” plants developed inflorescent shoots only under natural dayextension to 16 hours, with considerable effect of sun irradianceconditions on the rate of this development (the difference in lengthbetween the two growth periods).

As demonstrated above the critical day length of PHY A overexpressinglines was shorter than 10 hours and thus it is not surprising that theydeveloped inflorescent shoots under a photoperiod of 14 or 16 hours withno photoperiod effect on the developmental rate. Little sun irradianceeffect on the developmental rate, during the two growth cycles and underthe two photoperiods, was observed.

The PHY B overexpressing lines: AB12-6 and AB13-1 had critical daylength longer than 10 hours but shorter than 14 hours and therefore theydid not develop inflorescent shoots under a photoperiod of 10 hours, butdid so under a photoperiod of 14 hours. The effect of sun irradiance onthe rate of their inflorescent shoot development was higher under aphotoperiod of 14 than 16 hours. Under natural day extension to 16 hoursthe rates of inflorescent shoot development in transgenic PHY B lineswere only slightly affected by the natural day sun irradianceconditions, meaning that low sun irradiance during winter will have aslight effect on flowering in these lines under day extensionconditions.

Natural Day Extension with Fluorescent Lighting:

FIG. 3 demonstrates that in non-transgenic “Sun Karlo” plantsphytochrome plays an important role in determining the critical daylength. “Sun Karlo” plants responded not only to a photoperiod lengthand sun irradiance conditions, but also to the light spectrum during dayextension when the photoperiod was extended to the critical day lengthof these plants. When natural day was extended to 14 hours withincandescent lighting, the “Sun Karlo” plants did not developinflorescent shoots whereas extension with fluorescent lighting inducedthe development of inflorescent shoots. The developmental rate of theseshoots was highly influenced by sun irradiance conditions during the twogrowth cycles.

The PHY A overexpressing lines that were relatively insensitive to daylength longer than 10 hours showed little difference in their responseto day extension with incandescent or fluorescent lighting.

The PHY B overexpressing lines responded similarly to day extension ornight break treatments provided by fluorescent or incandescent lighting.

Night Break Treatment:

FIG. 4 demonstrates that two hours of night break with eitherincandescent or fluorescent lighting induced inflorescent shootdevelopment in “Sun Karlo” plants. The developmental rate of theseplants when treated with incandescent lighting was highly influenced bysun irradiance conditions whereas fluorescent lighting treatmentovercame the sun irradiance effect (almost no difference between thedevelopmental rate during the two growth cycles).

The transgenic PHY A or B overexpressing plants, which responded to theshort night break (15 minutes in the phytotrone, see above) where almostinsensitive to spectral content when night break was applied for twohours. Thus, the sun-irradiance effect on the developmental rate ofinflorescent shoots even under night break with incandescent lightingwas eliminated from PHY A overexpressing lines: AA20 and AA21 and PHY Boverexpressing lines: AB12-6 and AB13-1.

Example 5 Comparison of Commercial Parameters Regarding the Yield ofAster Transgenic Plants Grown in a Commercial Greenhouse DuringWintertime in Israel

PHY A or B transgenic lines and “Sun Karlo” non-transformed Aster plantswere grown in a commercial greenhouse for cut-flower production duringthe winter in Israel.

Growth conditions were as follows:

Natural short-day conditions were applied to the PHY A transgenic plantlines AA2-8 and AA4-7 while, natural short-day conditions extended byincandescent lighting to a photoperiod of 14 or 16 hours were appliedduring inflorescent shoot elongation to a “Sun Karlo” non-transformedAster plant, the PHY A transgenic line AA2-8 and the PHY B transgeniclines AB3-1 and AB12-6. In all cases, the inflorescent-shoot served asthe “cut-flower”. Cuttings of flowering shoots were performed at the endof each growth cycle. Three successive growth cycles were performedbetween autumn and spring in Israel.

The commercial quality of the flowering shoots, measured by their lengthand weight, was good and almost the same in all treatments; the numberof flowering shoots was highly affected by plant transformation.Overexpression of either, PHY A or B increased the number of floweringshoots mainly in the second and third growth cycles (Table 5 below).PHYA overexpression in the AA2-8 line increased the total shoot yield by51% without day extension and by 115% with minimum day extension. PHY Bexpressing lines AB3-1 and AB12-6 exhibited an increase in the totalshoot yield of 94% under day extension to 14 and 16 h, respectively.This commercial-yield test proves that transformation of Aster withPhytochrome encoding cDNA can increase the yield of flowering shoots andeliminate or reduce the need for artificial lighting when natural daysare short and during year-round production of “cut-flowers”. TABLE 5Flowering shoots (“cut flowers”) yield in wild type Aster cultivar “SunKarlo” and four transgenic lines grown under commercial conditions fromautumn through spring Flowering Flowering Photo- shoots Flowering shootsshoots Total shoot period First cycle second cycle third cycle yieldTotal Transgenic line hours No./m² No./m² No./m² No./m² shoots yield %Wild type 16 26 32 114 172 100 AA2-8 Natural 33 64 164 261 151 AA2-8 1428 92 251 371 215 AA4-7 Natural 33 41 141 215 125 AB3-1 14 39 75 222 336195 AB12-6 16 29 75 231 335 195Natural daylength changed during the production period between 13 h:40min and 10 h. Day extension was effected by incandescent lighting.During this period three growth cycles and flowering shoots cutting wereperformed. The four transgenic lines included the PHY A overexpressinglines AA2-8 and AA4-7, and the PHY B overexpressing lines AB3-1 andAB12-6.

Example 6 Generation and Analysis of Transgenic Hypericum cv. “ExcellentFlair” Plants

Plant Material:

Hypericum cv. “Excellent Flair” plants were grown in the phytotronecontrolled environment in order to keep the plants in their vegetativegrowth state. Growth conditions included short-day conditions of 10hours exposure to sun irradiance and controlled temperatures of 17:9° C.during day:night, respectively.

Plant Transformation:

Fully expanded leaves were surface sterilized in a 70% (v/v) ethanol forone minute followed by 10% (v/v) solution of domestic bleach for eightminutes. Cut leaf petioles were soaked in a 1/20 dilution of anovernight culture of the desired Agrobacterium strain containing eitherthe oat-PHY A-cDNA expression vector (pRFYI) or the Arabidopsis-PHYB-cDNA expression vector (pROKB) described hereinabove.

Infected petioles were placed onto MS salts plates, which contained halfconcentration of MS salts (0.5×MS) supplemented with 20g/l sucrose, 0.5mg/l naphthaleneacetic acid, 2.0 mg/l 6-benzylaminopurine and 7g/l agar.Plates were incubated under a constant temperature of 20° C. and 24hours cycles of 16 hours low light intensity from cool white fluorescentlamps followed by eight hours of dark.

Two days following transformation, the petioles were transferred tofresh MS salts plates containing 0.5×MS salts, 20g/l sucrose, 0.5 mg/lnaphthaleneacetic acid, 2.0 mg/l 6-benzylaminopurine, 7g/l agar, 100mg/l kanamycin and 400 mg/l augmentin and further incubated therein forthree weeks.

For shoot regeneration, the petioles were transferred onto fresh mediumcontaining 0.5×MS salt concentration, 20 g/l sucrose, 7 g/l agar, 0.5mg/l gibberellic acid, 1.0 mg/l 6-benzylaminopurine, 0.05 mg/lindolebutiric acid, 100 mg/l kanamycin and 400 mg/l augmentin.Regenerated shoots were excised and transferred onto fresh 0.5×MS platessupplemented with 20 g/l sucrose, 7 g/l agar, 100 mg/l kanamycin and 400mg/l augmentin).

The formation of roots in the excised shoots in the presence ofkanamycin was an indication that the plants have been successfullytransformed with the desired expression vector. Small plantlets (Shootswhich developed roots) of approximately 1.0 to 2.0 cm in length wereremoved from the media and transplanted in soil containing pots forhardening under phytotrone controlled conditions, which included highhumidity, short-days of 10 hours sun irradiance and controlledtemperatures of 20° C. day and 12° C. night.

Selection of Positive Transformed Hypericum Plants:

Leaves of hardened plants were PCR analyzed and the PCR positive plantswere further analyzed for expression of the exogenous cDNAs as describedhereinabove.

Positive selected plants were vegetatively propagated, using youngshoots as cuttings. These plants were grown under phytotrone controlledenvironment which included short-day conditions for rooting. Beforetesting the response of the Hypericum transgenic plants to day-length,three generations of plants originating from rooting shoots were grown.Each successfully grown plant generation represent a stable transformedplant expressing the relevant phytochrome coding sequences.

Cultivation Conditions:

Greenhouse/phytotrone: PHY A or PHY B transgenic plants of Hypericum cv.“Excellent Flair” were exposed to controlled environmental conditionseither in the phytotrone or the greenhouse. Phytotrone conditionsincluded temperature of 23:15° C., day:night, respectively, and shortnatural day illumination conditions of 10 hour exposure to sunirradiance (800 μmol m⁻²s⁻¹ PAR). These conditions prevented thenon-transgenic Hypericum plants from flowering and setting fruits. Inthe greenhouse a gradual increase in natural day-length starting inFebruary (10 h:40 min) and ending in June (14 hours), was utilized toselect early flowering plants.

Field: The transgenic Hypericum plants described above were also grownin the field under commercial growth conditions. Seedlings were plantedin the field on October 1 and cut-back to fifth internode on October 25.From October 25 to June 16, the plants were grown under lightingconditions which included a natural change in day-length as follows: 11h:06 min on October 25 decreasing to 10 h:03 min on December 21andincreasing to 14 h in June; or a natural day extension of up to 14 h viaincandescent lighting (0.7 μmol m⁻² s⁻¹ PAR).

Example 7 Flowering and Fruit Development in the Transgenic HypericumPlants

Data pertaining to flower development in the transgenic Hypericum, cv“Excellent Flair” plant lines is summarized in Table 6 below. TABLE 6Flowering of transgenic Hypericum, cv “Excellent Flair” transformed withoat-PHY A or Arabidopsis-PHY B coding sequences under short days in thephytotrone or greenhouse conditions Transgenic line Days to flowering inthe Days to flowering in the ID phytotrone greenhouse Excellent FlairDid not flower 139.0 ± 7.4 PHY A HA7 84.5 ± 0.6 108.3 ± 1.3 HA4 93.3 ±3.4 112.3 ± 2.1 HA16 86.3 ± 6.4 109.0 ± 2.5 HA25 95.6 ± 9.1 106.8 ± 1.2HA23 91.5 ± 2.8 111.1 ± 2.0 PHY B HB25 113.5 ± 3.0  108.4 ± 1.6 HB3193.5 ± 5.9 111.4 ± 3.6 HB16 99.0 ± 1.2 108.4 ± 3.1 HB19 98.5 ± 7.1 107.0± 5.4 HB42 84.3 ± 3.1 110.0 ± 2.3

As is evident from this data, the PHY A or PHY B transgenic Hypericumplants developed shoots and reached flowering under the short dayconditions provided in the phytotrone. In contrast, these conditionswere insufficient for the non-transgenic Hypericum cv. “Excellent Flair”plants, and as such these plants developed shoots but did not flower.

The transgenic and non-transgenic plants reacted differently to thegradual increase in day length provided by the greenhouse conditions.Although all of the rooted cuttings were planted in February, the PHY Aor PHY B transgenic plants lines flowered in May whereas thenon-transgenic plants flowered approximately one month later.

These results clearly demonstrate that flower initiation in thetransgenic Hypericum plant lines generated according to the teachings ofthe present invention occurs under day length conditions which aresubstantially shorter than that needed for flowering of similarnon-transgenic plants. This is clearly demonstrated under greenhouseconditions in which a gradual increase in day-length over time supportedflowering in the transgenic plant lines a full month before enablingflowering of identical but non-transformed plants.

These results are further demonstrated in FIG. 5, which illustrates thegrowth state of greenhouse grown transgenic and non-transgenic lines inJune (day length of approximately 14 hours). At this stage, transgenicplant fruits are clearly evident while the equivalent non-transgenicplants are still at their flowering stage necessitating an additionalgrowth period for fruit development.

In the field, flowering and fruit setting in transgenic Hypericum plantspreceded flowering and fruit setting of the wild type “Excellent Flair”plants (FIG. 7). Therefore phytochrome overexpression in Hypericumadvanced the appearance of fruit bearing shoots especially under naturalday-length conditions (FIGS. 8 a-g) and thus commercial yield oftransgenic Hypericum plants preceded that of the wild type plants bynearly one month.

Thus, the present invention provides two genetically distinct plantspecies which when over expressing phytochrome A or B respond in aqualitative manner to day length manipulation.

Aster (Asteraceae) cv “Sun Karlo” is an herbaceous perennial plantcultivated for its flowering shoot. Wild type Aster plant requireslong-day conditions for its inflorescent-shoot development, which on theother hand retards further flower development of the inflorescent shoot.As exemplified hereinabove, phytochrome overexpressing Aster plantsresponded to day length manipulation in a qualitative manner manifestedby induction of inflorescent-shoot development.

Hypericum (guttiferae) cv “Excellent Flair” is a woody perennial plant,cultivated for its decorative fruits. Wild type Hypericum requireslong-day conditions for flower initiation. As exemplified hereinabove,phytochrome overexpressing Hypericum plants responded to day lengthmanipulation in a qualitative manner manifested by flowering initiationunder substantially short day conditions.

In any case, plants generated according to the teachings of the presentinvention are particularly suitable for commercial cultivation sincethey respond, in flowering, to substantially shorter days thus enablingcommercial cultivation year round with little or no need for artificiallighting.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims. All publications, patents and patentapplications mentioned in this specification are herein incorporated intheir entirety by reference into the specification, to the same extentas if each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated herein byreference. In addition, citation or identification of any reference inthis application shall not be construed as an admission that suchreference is available as prior art to the present invention.

References (Additional References are Cited in the Text)

-   1. Bagnall D. J., King R. W., Whitelam G. C., Boylan M. T.,    Wagner D. and Quail P. H., 1995. Flowering response to altered    expression of phytochrome in mutants and transgenic lines of    Arabidopsis thaliana (L.) Heynh. Plant Physiol. 108, 1495-1503.-   2. Bernier G.,Kinet J. M. and Sachs R. M., 1981.The Physiology of    Flowering. CRC press, Boca Raton Fla.-   3. Casal J. J., Clough R. C. and Viestra R. D., 1996.    High-irradiance responses induced by far-red light in grass    seedlings of the wild type or overexpressing phytochrome A. Planta    200, 132-137.-   4. Goto N., Kumagai T. and Koornneef M., 1991. Flowering responses    to light-breaks in photomorphogenic mutants of Arabidopsis thaliana,    a long-day plant. Physiol. Plant. 83, 209-215.-   5. Halevy A. H., 1985. Handbook of Flowering. CRC press Boca Raton,    Fla.-   6. Johanson E., Bradley M., Harberd N. P. and Whitelam G. C., 1994.    Photoresponses of light grown phy-A mutants of Arabidopsis. Plant    physiol. 105, 141-149.-   7. Mathews S. and Sharrock R. A., 1997. Phytochrome gene diversity.    Plant, Cell and Environment 20, 666-672.Murtas G. and Millar A.    J., 2000. How plants tell the time. Current Opinion in Plant    Biology, 3, 43-46.-   8. Putterill J., Robson F., Lee K., Simon R. and Copland G., 1995.    The CONSTANT gene of Arabidopsis promotes flowering and encodes a    protein showing similarities to zink finger transcription factors.    Cell, 80, 847-857.-   9. Reed J. W., Nagpal P., Poole D. S., Furuya M. and Chory J., 1993.    Mutation in the gene for the Red/Far-red light receptor phytochrome    B alter cell elongation and physiological responses throughout    Arabidopsis development. The Plant Cell, 5, 147-157.-   10. Reed J. W., Nagatani A., Elich T. D., Fagan M. and Chory    J., 1994. Phytochrome A and phytochrome B have overlapping but    distinct functions in Arabidopsis development. Plant physiol. 104,    1139-1149.-   11. Samach A. and Copland G., 2000. Time measurements and the    control of flowering in plants. Review articles BioAssay 22, 38-47-   12. Somer D. E., Devlin P. F. and Kay S. A., 1998. Phytochrome and    cryptochromes in the entrainment of the Arabidopsis circadian clock.    Science 282, 1488-1490.-   13. Thomas B. and Vince-Prue D., 1995. Do long-day plants and    short-day plants perceive day length in the same way? FNL 20, 50-57.-   14. Thomas B. and Vince-Prue D., 1997. Photoperiodism in Plants.    Academic Press, London.-   15. Vince-Prue D, 1994. The duration of light and photoperiodic    responses. In: Photomorphogenesis in Plants. Kendrick R. E. and    Kronenberg G. H. M., eds. Kluwer, press Amsterdam. Pp. 447-490.-   16. Weller J. L., Murfet I. C. and Reid J. B., 1997. Pea mutants    with reduced sensitivity to Far-red light define an important role    for phytochrome A in day-length detection. Plant physiol. 114,    1225-1236.-   17. Whitelam G. C. and Harberd N. P., 1994. Action and function of    phytochrome family members revealed through the study of mutant and    transgenic plants. Plant Cell and Environment 17, 615-625.-   18. Whitelam G. C. and Devlin P. E., 1997. Roles of different    phytochrome in Arabidopsis photomorphogenesis. Plant, Cell and    Environment 20, 752-759.

1. A long day plant cultivated for commercial production offlowering-shoots, flowering pots, flowers, seeds or fruits, the long dayplant overexpressing a phytochrome protein in at least a portion of it'scells, such that said flowering-shoots, flowering pots, flowers, seedsor fruits thereof develop under substantially shorter days than thatrequired for development of said flowering-shoots, flowering pots,flowers, seeds or fruits in a similar long day plant not over expressingsaid phytochrome protein.
 2. The long day plant of claim 1, wherein saidsubstantially shorter days are effected by natural lighting conditions.3. The long day plant of claim 1, wherein said substantially shorterdays are further characterized by at least one condition selected fromthe group consisting of a light intensity between 80-2000 μmole m⁻²s⁻¹PAR, and a temperature selected from the range between 5-30° C.
 4. Thelong day plant of claim 1, wherein the long day plant overexpressingsaid phytochrome protein is derived from a commercial plant, plantderived tissue or a plant cell transformed with an exogenous expressioncassette for overexpressing said phytochrome protein.
 5. The long dayplant of claim 4, wherein said commercial plant, plant derived tissue ora plant cell is stably or transiently transformed with said expressioncassette for overexpressing said phytochrome protein.
 6. The long dayplant of claim 5, wherein said expression cassette forms a part of anucleic acid construct selected from the group consisting of a DNAconstruct or an RNA construct.
 7. The long day plant of claim 4, whereinsaid exogenous expression cassette includes a phytochrome A or aphytochrome B encoding sequences.
 8. The long day plant of claim 7,wherein said exogenous expression cassette also includes a promotersequence for directing expression of said phytochrome A or saidphytochrome B encoding sequences in plant tissue.
 9. The long day plantof claim 8, wherein said promoter is selected from the group consistingof a constitutive promoter, an inducible promoter a developmentallyregulated promoter and a tissue specific promoter.
 10. The long dayplant of claim 1, wherein the long day plant overexpressing saidphytochrome protein is a commercial dicotyledonous or monocotyledonousplant.
 11. The long day plant of claim 1, wherein the long day plantoverexpressing said phytochrome protein is selected from the groupconsisting of an agronomic crop, a horticultural crop, and an ornamentalplant.
 12. The long day plant of claim 1, wherein the long day plantoverexpressing said phytochrome protein is an annual or a perennialplant selected from the group consisting of a rosette forming plant, abulb forming plant, a corm forming plant, an herbaceous plant, a shrubforming plant and a tree forming plant.
 13. The long day plant of claim1, wherein said flowering-shoots, flowering pots, flowers, seeds orfruits which develop under said substantially shorter days have at leastone improved agronomic and/or commercial characteristic selected fromthe group consisting of an increased number of flowering shoots, anincreased number of flowers, an increased number of fruit formingflowers and a faster growth rate, lower cold request for growth andflowering, and reduced light intensity dependent flowering as comparedto said similar long day plant not over expressing said phytochromeprotein.
 14. The long day plant of claim 1, wherein a day length of saidsubstantially shorter days is at least 15% shorter than that required bysaid similar long day plant not over expressing said phytochromeprotein.
 15. A method of modulating a responsiveness of a long day plantto day length comprising: (a) overexpressing a phytochrome protein in aplurality of long day plants; and (b) selecting plants from saidplurality of long day plants which are capable of developingflowering-shoots, flowering pots, flowers, seeds or fruits when grownunder substantially shorter days than those required for development ofsaid flowering-shoots, flowering pots, flowers, seeds or fruits in asimilar long day plant not overexpressing said phytochrome protein,thereby modulating a responsiveness of the long day plant to day length.16. The method of claim 15, wherein said substantially shorter days areeffected by natural lighting conditions.
 17. The method of claim 15,wherein the long day plant is cultivated for commercial production offlowering-shoots, flowering pots, flowers, seeds or fruits.
 18. Themethod of claim 15, wherein the long day plant is derived from acommercial plant, plant derived tissue or a plant cell transformed withan exogenous expression cassette for overexpressing said phytochromeprotein.
 19. The method of claim 18, wherein said commercial plant,plant derived tissue or a plant cell is stably or transientlytransformed with said exogenous expression cassette for overexpressingsaid phytochrome protein.
 20. The method of claim 18, wherein saidexogenous expression cassette forms a part of a nucleic acid constructselected from the group consisting of a DNA construct or an RNAconstruct.
 21. The method of claim 18, wherein said exogenous expressioncassette for overexpressing said phytochrome protein includes aphytochrome A or a phytochrome B encoding sequences.
 22. The method ofclaim 21, wherein said exogenous expression cassette also includes apromoter sequence for directing expression of said phytochrome A or saidphytochrome B encoding sequences in plant tissue.
 23. The method ofclaim 22, wherein said promoter is selected from the group consisting ofa constitutive promoter, an inducible promoter, a developmentallyregulated promoter and a tissue specific promoter.
 24. The method ofclaim 15, wherein a day length of said substantially shorter days is atleast 15% shorter than that required by said similar long day plant notover expressing said phytochrome protein.
 25. The method of claim 18,wherein said exogenous expression cassette for overexpressing saidphytochrome protein is compatible for propagation in cells, orintegration into the genome, of a plant.
 26. The method of claim 19,wherein said stable and/or said transient plant transformation isfollowed by identifying said long day plants.
 27. The method of claim15, wherein said overexpressing is effected by transforming said longday plant with an expression cassette encoding a phytochrome protein.28. The method of claim 27, wherein said phytochrome protein isphytochrome A or phytochrome B.
 29. The method of claim 15, whereinmodulating the responsiveness of the long day plant to day length isutilized for producing flowering-shoots, flowering pots, flowers, seedsor fruits in the long day plant during substantially shorter days thanthose required by the long day plant for producing saidflowering-shoots, flowering pots, flowers, seeds or fruits.
 30. Themethod of claim 15, wherein modulating the responsiveness of the longday plant to day length is utilized for causing a spring or summerflowering plant to flower during autumn, winter or year-round.
 31. Themethod of claim 15, wherein modulating the responsiveness of the longday plant to day length is utilized for conferring early flowering inthe long day plant under short-day conditions.
 32. The method of claim15, wherein said substantially shorter days are further characterized byat least one condition selected from the group consisting of a lightintensity between 80-2000 μmole m⁻²s⁻¹ PAR, and a temperature selectedfrom a range between 5-30° C.
 33. A method of commercially cultivating along day plant under short day conditions, comprising: (a)overexpressing a phytohormone protein in at least a portion of the cellsof the long day plant; and (b) cultivating the long day plant resultingfrom step (a) under substantially shorter days than those required fordevelopment of flowering-shoots, flowering pots, flowers, seeds orfruits in a similar long day plant not overexpressing said phytochromeprotein, thereby commercially cultivating said long day plant under theshort day conditions.
 34. The method of claim 33, wherein saidsubstantially shorter days are effected by natural lighting conditions.35. The method of claim 33, wherein said long day plant is derived froma commercial plant, plant derived tissue or a plant cell transformedwith an exogenous expression cassette for overexpressing saidphytochrome protein.
 36. The method of claim 35, wherein said commercialplant, plant derived tissue or a plant cell is stably or transientlytransformed with said exogenous expression cassette for overexpressingsaid phytochrome protein.
 37. The method of claim 35, wherein saidexogenous expression cassette forms a part of a nucleic acid constructselected from the group consisting of a DNA construct or an RNAconstruct.
 38. The method of claim 35, wherein said exogenous expressioncassette for overexpressing said phytochrome protein includes aphytochrome A or a phvtochrome B encoding sequences.
 39. The method ofclaim 38, wherein said exogenous expression cassette further includes apromoter sequence for directing an expression of said phytochrome A orsaid phytochrome B encoding sequences in a plant tissue.
 40. The methodof claim 39, wherein said promoter is selected from the group consistingof a constitutive promoter, an inducible promoter, a developmentallyregulated promoter and a tissue specific promoter.
 41. The method ofclaim 33, wherein a day length of said substantially shorter days is atleast 15% shorter than that required by said similar long day plant notoverexpressing said phytochrome protein.
 42. The method of claim 35,wherein said exogenous expression cassette for overexpressing saidphytochrome protein is compatible for propagation in cells, orintegration into the genome, of a plant.
 43. The method of claim 36,wherein said stable and/or said transient plant transformation isfollowed by identifying said long day plants.
 44. The method of claim33, wherein said overexpressing is effected by transforming said longday plant with an expression cassette encoding said phytochrome protein.45. The method of claim 44, wherein said phytochrome protein isphytochrome A or phytochrome B.
 46. The method of claim 33, wherein saidcultivating is effected during autumn, winter or year-round.
 47. Themethod of claim 33, wherein said substantially shorter days are furthercharacterized by at least one condition selected from the groupconsisting of a light intensity between 80-2000 μmole m⁻²s⁻¹ PAR, and atemperature selected from a range between 5-30° C.
 48. The method ofclaim 33, wherein the short day conditions are natural short dayconditions.